1. Field of the Invention
The present invention relates to a control device for an alternating current motor. More specifically, the present invention relates to a technique used in an electric current feedback control system for carrying out a follow-up control of a measured value of the electric current with respect to an ordered value of the electric current.
2. Description of Related Art
In an alternating current (AC) motor, such as a permanent magnet type motor which utilizes a permanent magnet in the field, a control device for an AC motor is generally known which measures the electric current of an armature of the AC motor, converts the measured value into rectangular coordinates which rotate in synchronization with a rotor, i.e., the dq coordinate system, and carries out a feedback control so that the deviation between the ordered value and the measured value of the current on the dq coordinate becomes zero.
In such an AC motor, a change in the magnetic flux density is generated in the field magnetic flux which penetrates through a coil of the armature when, for instance, a rotor having a permanent magnet rotates, and a back electromotive voltage Er, which acts to cancel the supplied voltage of the coil electric current, is generated. The back electromotive voltage Er increases as the number of rotations of the rotor increases and, when Er becomes equal to the supplied voltage of the coil electric current, the coil electric current becomes zero and the rotation torque of the rotor also becomes zero.
A so-called weak field control is known, which makes it possible to increase, for instance, the operable range of the number of rotation, the rotation torque which may be generated, the number of rotations at which the motor can operate with high efficiency, and the range of rotation torque, by weakening the magnetic flux of the field equivalently.
FIG. 4 is a vector diagram showing a stationary state of an example of a conventional control device for an AC motor when a vector control is performed. In the figure, the direction of the magnetic flux of the field is indicated by the d-axis and the direction which is perpendicular to the d-axis is indicated by the q-axis. Ld and Lq indicate the inductance of the d-axis and the q-axis, respectively; R indicates an each phase resistance of the alternating current motor; xcfx89re indicates a velocity of the electrical angle of the AC motor; xcfx86 indicates a main magnetic flux of the field of the AC motor; id and iq indicate the electric current along the d-axis and the q-axis, respectively; vd and vq indicate the voltage along the d-axis and the q-axis, respectively, and Vmax indicates a maximum voltage which may be supplied to each phase of the AC motor.
In this case, the voltage, vd, in the d-axis and the voltage, vq, in the q-axis may be expressed by the equations (1) shown below. In the equation, xcfx89rexc3x97Ldxc3x97id, which is a q-axis interference term, becomes a weak field component when the back electromotive voltage Er exceeds the maximum voltage Vmax, which may be supplied to each phase of the AC motor, and a weak field control is performed. Accordingly, the vector of the q-axis interference term extends downwardly in FIG. 4 when the electric current id in the d-axis is increased and, hence, the AC motor may be actuated by using a voltage smaller than the back electromotive voltage Er=(xcfx89rexc3x97xcfx86) at the voltage vq in the q-axis. In this manner, a desired rotational torque may be output by increasing the number of operational rotations.
Vd=Rxc3x97idxe2x88x92xcfx89rexc3x97Lqxc3x97iq 
(where xcfx89rexc3x97Lqxc3x97iq indicates d-axis interference term) 
Vq=Rxc3x97iq+xcfx89rexc3x97xcfx86+xcfx89rexc3x97Ldxc3x97id xe2x80x83xe2x80x83(1) 
(where xcfx89rexc3x97Ldxc3x97id indicates q-axis interference term) 
In the above-mentioned example of the conventional control device for the AC motor, after the back electromotive voltage, Er, of the AC motor exceeds the maximum voltage, Vmax, which may be supplied to each phase of the AC motor, and the rotation number is further increased, the actuation of the AC motor becomes impossible if xcfx89rexc3x97Ldxc3x97id, which is the q-axis interference term and becomes a weak field component, is not increased in accordance with an increase in the back electromotive voltage Er=(xcfx89rexc3x97xcfx86).
In the state described above, the magnitude of the voltage vq at the q-axis tends to be dominated by the magnitude of the electric current id at the d-axis. Further, because the d-axis interference term: xe2x88x92xcfx89rexc3x97Lqxc3x97iq is present at the d-axis, the voltage vd on the d-axis tends to be dominated by the magnitude of the electric current iq on the q-axis.
In the above control device for an AC motor, however, the control of the electric current feedback is separately carried out for the d-axis and the q-axis. Hence, at the d-axis, the control is executed so that the deviation between the ordered value for the d-axis and the measured value of the electric current becomes zero and, at the q-axis, it is controlled so that the deviation between the ordered value for the q-axis and the measured value of the electric current becomes zero. For this reason, in a state in which one of the voltages vd and vq becomes dominant to the other voltage, there is a danger that the current control for stabilizing the AC motor may be destroyed. Thus, the current control may be destabilized and the electric current may be varied rapidly or a desired torque may not be obtained.
On the other hand, in the region where the back electromotive voltage Er of an AC motor is smaller than the maximum voltage, Vmax, which may be supplied to each phase of the AC motor, a so-called non-interactive control is known by which a d-axis compensation term and a q-axis compensation term that counteract each interference component for the d-axis and q-axis, respectively, are input so that an independent control of the d-axis and the q-axis becomes possible by counteracting the speed electromotive force components which interfere with each other between the d-axis and the q-axis.
However, in the region where the back electromotive voltage Er exceeds the maximum voltage Vmax, which may be supplied to each phase of the AC motor, the non-interactive control cannot be carried out since a power source, for instance, a battery or fuel cell, which supplies a voltage to the coil current, cannot provide the extra voltage. Thus, problems such as an instability in the control of the AC motor may be generated.
Accordingly, one of the objectives of the present invention is to provide a control device for an AC motor which can perform a stable control of the motor even when the back electromotive force of the AC motor is increased.
The above objectives may be achieved by a control device for an alternating current motor according to the present invention (for example, a control device 10 for an alternating motor explained in the embodiment described later), including: a target current generating unit (for instance, a target current computing unit 22 in the embodiment described later) which generates a current order value, based on a torque order (for example, a torque order, *T, in the embodiment described later), as a d-axis target current (for example, a d-axis target current, *id, in the embodiment described later) and a q-axis target current (for example, a q-axis target current, *iq, in the embodiment described later) on dq coordinates which are of a rotating rectangular coordinate system; a current detection device (for example, electric current detectors 16 and 17 in the embodiment described later) which detects an alternating current supplied to each phase (for example, a U-phase, V-phase, and W-phase in the embodiment described later) of a polyphase alternating current motor (for example, an AC motor 11 in the embodiment described later); a coordinate transforming unit (for example, a three-phase ac-dq coordinate transformer 31 in the embodiment described later) which transforms the alternating current detected by the current detection device into a d-axis current (for example, a d-axis current, id, in the embodiment described later) and a q-axis current (for example, a q-axis current, iq, in the embodiment described later) on the dq coordinates; and a vector control unit (for example, a vector controlling unit 23 in the embodiment described later) which carries out a current feedback control so that the d-axis current follows up the d-axis target current and the q-axis current follows up the q-axis target current, wherein the vector control unit further includes: an operation switching unit (for example, an integration operation switching unit 37 in the embodiment described later) which, depending on whether a back electromotive voltage (for example, a back electromotive voltage, Er, in the embodiment described later) of the alternating current motor is greater than or equal to a predetermined value, calculates one of a d-axis deviation (for example, a d-axis deviation, xcex94id, in the embodiment described later) and a q-axis deviation (for example, a q-axis deviation, xcex94iq, in the embodiment described later) from the deviation between the d-axis target current and the d-axis current (for example, a deviation, xcex94idorg, in the embodiment described later) and the other one of the d-axis deviation and the q-axis deviation from the deviation between the q-axis target current and the q-axis current (for example, a deviation, xcex94iqorg, in the embodiment described later); and an integration controlling unit (for example, integration control units 38 and 39 in the embodiment described later) which outputs a d-axis integration voltage order value (for example, a d-axis integration voltage order value, Vdi, in the embodiment described later) proportional to an integral value of the d-axis deviation and a q-axis integration voltage order value (for example, a q-axis integration voltage order value, Vqi, in the embodiment described later) proportional to an integral value of the q-axis deviation, wherein an alternating current supplied to each phase of the alternating current motor is controlled based on an output of the integration controlling unit.
According to the control device for an AC motor having the above-mentioned structure, it becomes possible, under a weak field control, to control the electric current of the AC motor in a stable manner even when the back electromotive voltage of the AC motor exceeds the maximum voltage, which may be supplied to each phase of the AC motor, and the d-axis voltage is dominated mainly by the q-axis current, iq, and the q-axis voltage is mainly dominated by the d-axis current, by carrying out an integration operation of an ordered voltage based on the current deviation of the opposite axis. Thus, according to the present invention, a desired torque may be assuredly generated.
The present invention also provides a control device for an alternating current motor, further including: a power conversion unit (for example, an inverter 13 in the embodiment described later) which drives the alternating current motor, the power conversion unit being controlled by the vector control unit; and a power supply unit (for example, a power supply 14 in the embodiment described later) which supplies a direct current to the power conversion unit, wherein the predetermined value is equal to the maximum voltage which may be supplied to the alternating current motor from the power conversion unit (for example, the maximum voltage, Vmax, which may be supplied to the AC motor 11 from the inverter 13 in the embodiment described later) and adjustable depending on the magnitude of a voltage (for example, a power supply voltage, Vdc, in the embodiment described later) supplied by the power supply unit.
According to the control device for an AC motor having the above-mentioned structure, for instance, when the voltage of the power supply unit is changed, it is possible to assuredly obtain the timing at which an integration operation of an ordered voltage based on the current deviation of the opposite axis is carried out. Accordingly, for example, when the number of rotations of the AC motor is low, the integration operation of the ordered voltage can be carried out in a secured manner based on the current deviation of the opposite axis in synchronization with the start timing of the weak field control. Thus, the AC motor can be smoothly controlled.